When Daniel Pearl, a correspondent for the Wall Street Journal, was abducted in Karachi, beheaded and his murder filmed, Muslim extremists signalled that they had added a new and shocking tactic to their methods.

The Chechens used to make similar videos of captured Russian soldiers, but Pearl was the first foreigner to be killed in such a manner and his murder in Pakistan five years ago marked the beginning of a resurgence of beheading, a historic Islamic practice, as a means of terror.

Insurgents in Iraq soon adopted it. In 2004, a video posted on a militant website linked to al-Qaida, showed the killing in Iraq of a young American called Nick Berg. His body was discovered beside a Baghdad road a month after he vanished.

The footage showed Mr Berg, who was seeking communications work in Iraq, surrounded by five men wearing headscarves. He is heard saying: "My name is Nick Berg, my father's name is Michael, my mother's name is Susan. I have a brother and a sister, David and Sarah. I live in Philadelphia."

After his killing, one of the attackers read a statement saying the death was to redeem the dignity of prisoners in Abu Ghraib jail, the centre of the abuse allegations against American forces.

A wave of kidnappings and ritual beheadings followed the death of Mr Berg, with video and the internet used to distribute the images across the world.

Terrorists were quick to understand the power of such a single act. What developed was a market in brutality, with criminal gangs abducting foreigners and selling them on to terrorist groups such as the Jordanian Abu Musab al-Zarqawi's Tawhid and Jihad.

Al-Zarqawi became synonymous with some of the worst atrocities, and is believed to have personally beheaded Mr Berg, the US contractor Eugene Armstrong and the British contractor Ken Bigley.

Many captives, such as Mr Bigley and later the Irish-born charity worker Margaret Hassan, were kept alive for weeks, leading to coverage in the western press.

The coverage led to warnings by Ayad Allawi, Iraq's US-appointed interim prime minister, among others that the media were playing along with the kidnappers by giving the hostage-takers such publicity.

In November 2004, US marines came across what appeared to be an improvised television studio, with video cameras, banks of computers and editing equipment, and a wall spattered with blood and draped with the insignia of an Islamic militant group believed to be behind the killings of dozens of hostages in Iraq.

The fad in terrorist brutality has spread to Saudi Arabia, where beheading is used to punish crimes ranging from drug running to apostasy. Islamist terrorists in the country murdered an American businessman, Paul Johnson, whose head was later discovered in a freezer.

By last year, insurgents in Iraq had kidnapped more than 250 foreigners and killed at least 40 of them.